ABSTRACT
There are two main problem conditions. One occurs where valves are used to control the flow in a system by creating a pressure drop, e.g. a trimming valve. In other situations, valves normally operate either fully open or fully shut, in which case problems only occur when the valve vents to atmosphere, as in the case of an emergency dump valve, or when it is necessary to dump unwanted gas or air possibly at an intermediate stage during plant start up.
